The invention relates to a multicarrier frequency hopping spread spectrum communications system comprising at least one transmitter station and at least one receiver station communicating with each other by at least one transmit channel, the system comprising, at the transmitting end, baseband transfer means for transferring data by frequency hopping spread spectrum coding and, at the receiving end, reverse transfer means for recovering the transmitted data.
The invention likewise relates to a communications system comprising a transceiving base station and a plurality of transceiving user stations. The invention also relates to a transceiving user station, specifically either a mobile station communicating radio channel, or a fixed station communicating by cable, satellite or other means. The invention also relates to a base station to receive and relay messages with a view to having mobile stations communicate with each other.
A communications system for communications between users is to satisfy various criterions, and among these criterions the criterion of confidentiality is frequently experienced. The designers of such systems have thus moved towards the use of coding for message transmission. A high performance coding system is the Code-Division Multiple Access CDMA system. This system provides the advantages of reliability, robustness to disturbance, robustness to interference or degradation and can, furthermore, readily be tailored to any number of users. The principle of the CDMA system consists of spreading the frequency spectrum. Therefore, a coding message of a first duration Ts is transformed into a message of a first duration To, so that Tc=Ts/1. The parameter one may be smaller than, equal to or greater than 1. When there are M users at a given instant, there are M messages of duration Tc. These messages are then further coded with the known technique called frequency hopping spread spectrum coding, which consists of modulating M different carriers with the M messages and switching from time to time the assignment of each carrier to each respective user to distribute the transmission errors of the channel. When 1 is greater than one the fast frequency hopping technique is obtained, and when 1 is smaller than one, the slow frequency hopping technique is obtained. When 1 is equal to one, the duration of the message to be coded is not changed.
Such a system is described, for example, in the text by J. G. PROAKIS, "Digital Communications", chapter 8, pages 845-860, published by MCGRAW-HILL Book Company (1989), New York.
A base station receives the coded messages coming from all the users, decodes them, then recodes and relays them to the receiving users. The station supervises the communications, that is to say, it allows a receiving user and him alone to decode a message destined for that user. For such a system it is necessary to use an equalizer at the receiving end, which equalizer may become complex when the number in users grows. In effect, when there are echo signals of a multi-path environment, the echo signals of the channel of one user will generally not be the same as the echo signals of the channel of another user. The real performance of such a system is thus limited. Also, it is necessary to synchronize the receiver at the start of each hopping sequence, so that the received message can be decoded, which often is a problem.
In such a system it is necessary to use a frequency generator to generate the carder assigned to each user. This generator must be able to change frequency rapidly with each frequency jump. To avoid overlap of the frequencies of two generators, it is generally necessary to provide a guard band, which is a handicap.